Various endoprostheses assemblies which include expandable stents have been proposed or developed for use in association with angioplasty treatments and other medical procedures. The endoprosthesis assembly is percutaneously routed to a treatment site and the stent is expanded to maintain or restore the patency of a body passageway such as a blood vessel. A stent is typically cylindrical in shape comprising an expandable, open frame which radially expands upon exertion of an outwardly directed radial force on an inner surface of the stent frame.
Stents for endovascular implantation into a blood vessel, artery or the like to maintain or restore the patency of the passageway have been deployed percutaneously to minimize the invasiveness associated with surgical exposure of the treatment site. Percutaneous deployment is initiated by an incision into the vascular system of the patient, typically into the femoral artery. A tubular or sheath portion of an introducer is inserted through the incision and extends into the artery. The introducer has a central lumen which provides a passageway through the patient's skin and artery wall into the interior of the artery. An outwardly tapered hub portion of the introducer remains outside the patient's body to prevent blood from leaking out of the artery along the outside of the sheath. The introducer lumen includes a valve to block blood flow out of the artery through the introducer passageway. A distal end of a guide wire is passed through the introducer passageway into the patient's vasculature. The guide wire is threaded through the vasculature until the inserted distal end extends just beyond the treatment site. The proximal end of the guide wire extends outside the introducer.
For endovascular deployment, a stent, in an unexpanded or constricted configuration is crimped onto a deflated balloon portion of a balloon catheter. The balloon portion is normally disposed near a distal end of the balloon catheter. The catheter has a central lumen extending its entire length. The distal end of the balloon catheter is threaded onto the proximal end of the guide wire. The distal end of the catheter is inserted into the introducer lumen and the catheter is pushed along the guide wire until the stent reaches the treatment site. At the treatment site, the balloon is inflated causing the stent to radially expand and assume an expanded configuration. When the stent is used to reinforce a portion of the blood vessel wall, the stent is expanded such that its outer diameter is approximately 10% to 20% larger than the inner diameter of the blood vessel at the treatment site, effectively causing an interference fit between the stent and the blood vessel that inhibits migration of the stent. The balloon is deflated and the balloon catheter is withdrawn from the patient's body. The guide wire is similarly removed. Finally, the introducer is removed from the artery.
The introducer provides a passageway from the external environment into the patient's vascular system. The size of the cross sectional area of the introducer sheath portion determines the required size of the entrance incision or wound into the vascular system. Reduction in the required size of the introducer passageway, which is typically measured in terms of "French size," permits an introducer with a smaller outer diameter to be used and results in a correspondingly smaller entrance incision into the patient's vascular system. A smaller entrance incision reduces the probability that surgical closure of the wound will be necessary and reduces the risk of developing hematomas and other bleeding complications. However, efforts to reduce the required size of the introducer passageway are limited by the deployment cross sectional profile of the endoprosthesis assembly as mounted on the balloon portion of the catheter.
Typically, a stent consists of two or more struts or support members connected together into a radially expandable frame. The struts define open or void areas upon expansion of the stent from the constricted configuration to an expanded configuration at the treatment site. Larger void areas can permit malignant tissue growth through the stent openings into the body passageway and can also allow undesired contact between blood in the blood vessel and damaged portions of the vessel wall. Stent covers have been proposed to alleviate the problems associated with stent openings. Unfortunately, conventional stent covers greatly increase the cross sectional profile of the endoprosthesis assembly, necessitating the use of a larger sized introducer and requiring a correspondingly larger entrance incision.
Further, as disclosed in co-pending application Ser. No. .sub.------, filed Jun. 27, 1996, and entitled Controlled Porosity Endovascular Implant (Atty. Docket No. 13-949), assigned to the assignee of the present invention and is incorporated herein by in its entirety by reference, while the stent cover functions to prevent malignant tissue growth through the stent openings, some porosity in the stent cover is desirable. The stent cover should have pores of sufficient size to allow cellular ingrowth and capillary formation but small enough to prevent intrusions into the stent passageway as discussed above.
One proposed prior art stent cover comprises a tubular sleeve designed to be mechanically deformed around a constricted stent by folding or bending the sleeve into a layered or bunched configuration overlying the stent. The sleeve is affixed to the stent by stitching or gluing. The resulting configuration is characterized by an uneven, bulky profile for the endoprosthesis assembly. Such a configuration compromises easy insertion of the endoprosthesis assembly through the introducer passageway and necessitates a larger diameter passageway.
Other proposed stent cover designs require the use of a deployment sheath to maintain the stent cover in a collapsed configuration about the stent during insertion and deployment. When the stent is at the treatment site, the deployment sheath is withdrawn prior to inflation of the balloon. A problem with using a deployment sheath is that it increases the deployment cross sectional profile of the assembly, which, as explained above, requires use of a larger French size introducer.
There remains a need for an endoprosthesis assembly including a stent cover that does not significantly increase the deployment cross sectional profile of the assembly. There further remains a need for an endoprosthesis assembly including a stent cover wherein the assembly can be percutaneously deployed without the necessity of a deployment sheath to maintain the cover in a wrapped configuration about the stent during deployment of the assembly. Also, there remains a need for an endoprosthesis assembly including a stent cover that prevents undesirable tissue growth through the stent openings yet provides sufficient porosity for desirably cellular ingrowth and capillary formation. Finally, there remains a need for an endoprosthesis assembly stent covering providing the above advantages and that can be used with existing stents.